The present invention relates to a method for the preparation of spherical particles of a rare earth phosphate or, more particularly, to a method for the preparation of a rare earth phosphate powder having a spherical particle configuration capable of giving an advantage of removing the difficulty encountered in the recovery of the precipitates of the phosphate from the precipitation medium by filtration and suitable as a base material of a rare earth phosphate-based phosphor useful mainly as a green-emitting phosphor in fluorescent lamps.
As is well known, cerium orthophosphate activated with terbium is a green-emitting phosphor and proposals have been made in Japanese Patent Kokai No. 60-10065 and No. 59-179578 and Japanese Patent Publication No. 1-41673 for a green-emitting rare earth phosphate-based phosphor containing one or more of the rare earth elements selected from yttrium, lanthanum and gadolinium in addition to cerium and terbium. It is generally accepted that the light-emitting efficiency of a rare earth-based phosphor is high when the particle configuration of the phosphor particles are as close to spherical as possible so that various proposals have been made heretofore in order to obtain spherical particles of a phosphor powder. For example, Japanese Patent Kokai No. 4-130014 pro- poses a method in which a powder of a rare earth phosphate as a base material of a rare earth-based phosphor is slurried in a suitable dispersion medium and the slurry is spray-dried by using a spray drier. This spray-drying method, however, is defective because the spherical particle obtained by this method is an agglomerate of a large number of fine primary particles so that the mechanical strength of such an agglomerate particle cannot be high enough to be readily disintegrated into the original fine particles resulting in poor workability in processing into a phosphor and decrease in the yield of the phosphor therefrom.
Apart from the problem of the light-emitting efficiency of the rare earth phosphate-based phosphor, one of the serious problems in the preparation of a rare earth phosphate powder by the method of precipitation is that the recovery of the phosphate precipitates from the precipitation medium by filtration is a very time-consuming process. Accordingly, it is eagerly desired to improve the filtrability of precipitates of a rare earth phosphate in order to increase the productivity of the process.